Optically variable devices are used in a wide variety of applications, both decorative and utilitarian. These devices can be made in variety of ways to achieve a variety of effects. Examples of optically variable devices include the holograms imprinted on credit cards and authentic software documentation, color-shifting images printed on banknotes, and enhancing the surface appearance of items such as motorcycle helmets and wheel covers.
Optically variable devices can be made as film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigment is commonly called a color-shifting pigment because the apparent color of images appropriately printed with such pigments changes as the angle of view and/or illumination is tilted. A common example is the “20” printed with color-shifting pigment in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Some anti-counterfeiting devices are covert, while others are intended to be noticed. Unfortunately, some optically variable devices that are intended to be noticed are not widely known because the optically variable aspect of the device is not sufficiently dramatic. For example, the color shift of an image, printed with color-shifting pigment, might not be noticed under uniform fluorescent ceiling lights, but more noticeable in direct sunlight or under single-point illumination. This can make it easier for a counterfeiter to pass counterfeit notes without the optically variable feature because the recipient might not be aware of the optically variable feature, or because the counterfeit note might look substantially similar to the authentic note under certain conditions.
Optically variable devices can also be made with magnetic pigments that are aligned with a magnetic field after applying the pigment, typically in a carrier such as an ink vehicle or a paint vehicle, to a surface. However, painting with magnetic pigments has been used mostly for decorative purposes. For example, use of magnetic pigments has been described to produce painted cover wheels having a decorative feature that appears as a three-dimensional shape. A pattern was formed on the painted product by applying a magnetic field to the product while the paint medium still was in a liquid state. The paint medium had dispersed magnetic non-spherical particles that aligned along the magnetic field lines. The field had two regions. The first region contained lines of a magnetic force that were oriented parallel to the surface and arranged in a shape of a desired pattern. The second region contained lines that were non-parallel to the surface of the painted product and arranged around the pattern. To form the pattern, permanent magnets or electromagnets with the shape corresponding to the shape of desired pattern were located underneath the painted product to orient in the magnetic field non-spherical magnetic particles dispersed in the paint while the paint was still wet. When the paint dried, the pattern was visible on the surface of the painted product as the light rays incident on the paint layer were influenced differently by the oriented magnetic particles.
Similarly, a process for producing of a pattern of flaked magnetic particles in fluoropolymer matrix has been described. After coating a product with a composition in liquid form, a magnet with desirable shape was placed on the underside of the substrate. Magnetic flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original planar orientation. This tilt varied from perpendicular to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflected incident light back to the viewer, while the reoriented flakes did not, providing the appearance of a three dimensional pattern in the coating. Although it is more common to align magnetic flakes, dielectric flakes can also be aligned in a similar manner to magnetic flakes by placing the dielectric flakes in an electric field.
While these approaches describe methods and apparatus for formation of three-dimensional-like images in paint layers, they are not suitable for high-speed printing processes because they are essentially batch processes. It is desirable to provide methods and apparatus for a high-speed in-line printing and painting that re-orients magnetic pigment flakes. It is further desirable to create more noticeable optically variable security features on financial documents and other products.
U.S. Pat. No. 7,047,883 in the name of Raksha et al., incorporated herein by reference, discloses a method and apparatus for orienting magnetic flakes. In this patent a high-speed system is disclosed wherein flakes in a UV curable binder on a moving web are aligned and subsequently cured using a UV-light source. In a particular embodiment this patent describes fixing the flakes before they pass over the trailing edge of the magnet by providing a UV source part way down the run of the magnet, for UV-curing carrier, or a drying source for evaporative carriers, for example. The drier disclosed within U.S. Pat. No. 7,047,883 incorporated herein by reference, is heater, for example, or in the instance that the ink or paint is a UV-curable, a UV lamp is used to cure the ink or paint. In another United States patent to Argoitia et al., UV curable ink or paint was disclosed and a UV lamp was used to cure magnetically aligned flakes within the ink or paint. U.S. Pat. No. 7,604,855 incorporated herein by reference also teaches that it is preferable to cure aligned flakes before leaving the trailing edge of a magnet on a moving substrate. Heretofore, large UV lamps have been used to cure magnetically aligned flakes in a UV curable binder. While these heaters and UV lamps serve an intended purpose, they are bulky and do not provide a way in which flakes in a binder within adjacent regions can be selectively cured.
It is an object of this invention to provide a method whereby high-speed inline printing and or painting that reorients magnetic flakes in a selected region and preserves their orientation is achieved while a web or substrate is moved at a relatively high speed to provide an optically variable device. The flakes which are oriented by the magnetic field are in a region that may form indicia such as a logo or the like, or may be surrounding indicia to highlight indicia on the substrate.
It is an object of this invention to provide in a preferred embodiment two distinct visible regions of aligned flakes wherein the alignment in each of the two regions is different from the other.
It is an object of this invention to first cure a first group of flakes with a moving laser beam and then to use other means for curing a remaining portion of flakes adjacent to the first group on a substrate.